1. Field of the Invention
The present invention relates to methods and apparatus for linear demodulation of fiber optic interferometric sensor signals. More particularly, this invention pertains to an improved synthetic heterodyne demodulator circuit.
2. Description of the Prior Art
Primarily two methods have been suggested by the prior art for the linear demodulation of fiber optic sensor signals. The first of these, "synthetic heterodyne" is described in U.S. patent Ser. No. 4,436,425 of James H. Cole entitled "Signal Waveform Detector Using Synthetic FM Demodulation" and in the article of James H. Cole, B.A. Danver and J.A. Bucaro entitled "Synthetic-Heterodyne Interferometric Demodulation", IEEE J. Quantum Elec., QE-18 694-697, April 1982. The other method, "Passive Homodyne" is discussed in the article of A. Dandridge, A.B. Tveten and T.G. Giallorenzi entitled "Homodyne Demodulation Scheme For Fiber Optic Sensors Using Phase Generated Carrier", IEEE J. Quantum Elec., QE-18, 1647-1653, October 1982.
Both of the above-described methods process a signal of the form: EQU s(t)=1+.gamma.cos (B sin .omega..sub.m t+.psi.(t)) (1)
such that the information of interest .psi.(t) is recovered with high fidelity. (Various methods are known for generating the above-described carrier signal in a fiber optic interferometric sensor).
In the passive homodyne demodulation approach, the carrier is first multiplied by synchronous reference signals x(t)=sin .omega..sub.m t, and y(t)=cos 2.omega..sub.m t. The pair of signal products are then lowpass filtered, resulting in baseband signals proportional to the sin .psi.(t) and cos .psi.(t), respectively. This quatrature pair of signal components is then differentiated, cross-multiplied and differenced, resulting in a signal proportional to the time derivative of .psi.(t). After integration, the desired signal .psi.(t) is produced.
In the synthetic heterodyne method, the carrier is filtered with a pair of bandpass filters centered at .omega..sub.m and 2.omega..sub.m. The bandpass filter outputs are then multiplied by synchronous reference signals of the form cos 2.omega..sub.m t and sin .omega..sub.m t, respectively. The resulting products, each of which contains a component at 3.omega..sub.m t, are then filtered with bandpass filters centered at 3.omega..sub.m t and then summed. This process results in a conventional phase-modulated (PM) carrier at 3.omega..sub.m t, which is then demodulated using standard FM demodulation techniques followed by integration. The entire signal processing chain results in linear recovery of .psi.(t), the signal of interest.
Both of the known methods possess serious shortcomings. The passive homodyne circuit requires two differentiators, four analog multipliers, two lowpass filters, a summer and an integrator. Furthermore, the scale factor of that circuit will be directly proportional to the amplitude of the incoming carrier unless automatic gain control (AGC) is provided. Generally, the AGC is placed at the input, prior to the first pair of multipliers. Either direct rectification (diode AM envelope detector) or a like method is employed to measure the amplitude of the incoming carrier. Means are then provided for adjusting the carrier to a predetermined peak level prior to processing. An additional multiplier is usually required to control the carrier level.
The two differentiator stages must be stable and well-matched. Otherwise, the demodulation process will become non-linear. Differentiator circuits can be tricky to design, particularly when wide bandwidths are required.
The synthetic heterodyne method requires four bandpass filters, two multipliers, a summer, an FM demodulator and an integrator. (Assuming that the FM demodulator incorporates some form of hard limiter at the front end, a separate AGC block is not required to remove scale factor fluctuations caused by variations in carrier amplitude.)
The bandpass filter requirement includes: one filter at frequency .omega..sub.m, one at frequency 2.omega..sub.m and two filters at frequency 3.omega..sub.m. Although the design of each of such filters is straightforward, this requirement adds to circuit complexity and cost. The two filters at 3.omega..sub.m must be well-matched and must track over temperature or nonlinearities will appear in the demodulation process. The gain stabilities of the filters at .omega..sub.m and 2.omega..sub.m are also critical since any variation will result in unbalanced contributions from the quadrature components to the synthesized FM sidebands.
An additional shortcoming of the synthetic heterodyne method is the requirement that the FM carrier frequency be fixed at three times the original carrier frequency. This may preclude operation at FM carrier frequencies where performance is optimal. It may also result in crosstalk problems since the FM demodulator is operating at the third harmonic of a frequency that is present at high levels elsewhere in the circuit.